This application relates to air filtering inside clean environments.
Air filtering is critical within environments that must remain clean, such as semiconductor device manufacturing environments. Tremendous efforts are made to eliminate process-limiting contaminants from the semiconductor device manufacturing site, commonly referred to as a clean room. Clean room contaminants may be generally classified as either particulate or molecular. Common particulate contaminants include dust, lint and other debris. Examples of process-limiting molecular contaminants include: acids, such as hydrochloric acid, nitric acid, phosphoric acid, hydrobromic acid; bases, such as ammonia, ammonium hydroxide, tetramethylammonium hydroxide, trimethylamine, triethylamine, hexamethyldisilazane, NMP, cyclohexylamine, diethylaminoethanol, methylamine, dimethylamine, ethanolamine, morpholine, condensables, such as silicones and hydrocarbons with a boiling point greater than or equal to 150.degree. C.; and dopants, such as boron (usually as boric acid), phosphorus (usually as organophosphate), and arsenic (usually in the form of arsenates).
Most of the efforts in clean room design have focused on removing particulate contaminants from the constituent airstreams, because particulate contaminants were viewed as having the most impact on device yields and device performance. Recently, it has been realized that molecular contamination can impose severe limitations on further reduction of device geometry and improvement of device performance.
Molecular air contaminants are actually collections of molecules, unlike particulate contaminants, and are most easily distinguished from particulates by size. Very small particulate matter may be about 1200 angstroms in diameter, while molecular contaminants are typically only a fraction of an angstrom in diameter (about 30,000-40,000 times smaller than typical particulate contaminants). This size differential translates into entirely different removal mechanisms for molecular and particulate contaminants. Two common molecular contaminant removal techniques are adsorption/condensation and chemisorption.
Unlike particulate matter, molecular air pollutants possess specific chemical and physical properties unique to the chemical specie they represent. The boiling point, vapor pressure, and reactivity characteristics of the molecular pollutants are especially important in the design of molecular air purification equipment. Generally, molecular contaminants with a boiling point of 100.degree. C. or greater may be effectively removed using activated carbon alone (by adsorption/condensation mechanisms), while removal of contaminants with lower boiling points (e.g., organophosphate, ammonia and other reactive amines) requires some sort of chemisorption mechanism (e.g., chemically treated activated carbon), in which the molecular contaminant and the reagent react to form a solid by-product at the surface of the activated carbon.
The chemisorption mechanisms available for use inside clean environments has been limited by the need to select chemical reagents that do not contribute contamination to the airstreams (i.e., the reagents and their by-products are solids under normal operating conditions). This has prevented the use of reagents that are highly effective at removing especially troublesome molecular contaminates that impose severe limitations on semiconductor device processes.
As used herein, the "normal operating conditions" of a clean environment are characterized by temperatures between 68.degree. F. and 70.degree. F. and a relative humidity of 40%.